The present invention relates to a wire matrix print head for use in printing apparatus and more particularly to an improved version of such a print head which is adapted for high speed operation while maintaining accurate alignment of its operating parts to assure reliability during use.
A number of print heads of the type contemplated by the present invention have been disclosed in the prior art. Such devices, which are commonly termed wire matrix print heads or mosaic print heads, have been employed for some time in teleprinters and the like and more recently in data processing equipment where high speed operation is particularly important.
In such devices, characters are printed in dot matrix form by a plurality of print wires which are respectively operated by separate electromagnetic units so that selected combinations of the wires are driven into a print position as the print head moves across the page or other surface upon which the printing is being performed.
Relatively early examples of wire matrix print heads have been disclosed by U.S. Pat. Nos. 3,333,667 issued Aug. 1, 1967 to Nordin; 3,828,908 issued Aug. 13, 1974 to Schneider and 4,009,772 issued Mar. 1, 1977 to Glaser, et al. These patents are typical of prior art relating to such print heads and illustrate that these print heads tend to operate in the same general manner. The printing head is passed line by line over the page or sheet to which the printing is to be applied. the electromagnetic units are operated by suitable circuitry of similar design and function so that the respective wires are shifted in various combinations between print and non-print positions. As the wires are shifted toward their print positions, they act upon the paper through a ribbon or the like in order to apply a matrix of dots upon the paper. This matrix of dots creates characters in the form of letters, numbers, or other selected symbols.
With the more recent demands for reliable and high speed operation of such print heads in data processing equipment and the like, it has become increasingly important to precisely control relative operation and movement of the electromagnetic units, the armatures and the print wires within the print head to produce uniform quality characters in printing.
At the same time, it is also important to provide reliable means for economically producing and assembling the print heads while making them capable of reliable high speed operation over extended periods of time. In order to provide rapid and accurate reaction of the armatures and print wires to operation of the respective electromagnetic units, the electromagnetic units are typically arranged in circumferential fashion with the armatures extending radially inwardly for interaction with an axially extending circumferential arrangement of print wires held by suitable guides.
With such an arrangement, relatively limited pivoting movement of the armature results from energization of the respective electromagnetic units. Each armature is thereupon pivoted from a rest or non-print position toward a print position so that it strikes a corresponding print wire and urges the print wire into a print position in order to produce characters by dot matrix formations in the manner described above.
When each electromagnetic unit is thereafter de-energized, both the print wire and the respective armature are returned to their rest or non-print positions, typically by means of a spring acting upon each print wire.
More recent versions of such wire matrix print heads adapted for use in data processing equipment and the like are disclosed for example in U.S. Pat. Nos. 4,051,941, 4,185,929 and 4,230,412 issued to Hebert on Oct. 4, 1977, Jan. 29, 1980 and Oct. 28, 1980, respectively. Each of these patents disclosed a print head of the type summarized above. These patents are also representative of efforts to satisfy the requirements of high speed operation and reliability in such print heads. The last noted patent in particular disclosed such a print head including a combination of print wires, armatures and respective electromagnetic units operable in the manner disclosed above and contained and supported within a housing formed of multiple portions which are secured together in order to provide operating alignment between the various components. In particular, one housing portion of the last noted patent was adapted to mount the electromagnetic units and another portion provided the axial guides for the print wires. The armatures were captured in proper orientation between the respective print wires and electromagnetic units upon assembly of the housing portions. Critical spacing between the operating components was established by means of a threaded fastener extending axially through the center of one housing portion for attachment to the other portion. In addition to securing the two housing portions in engagement with each other, the threaded fastener also serves to provide adjustment for a resilient O-ring member which functioned to limit return of the armatures and print wires from their print positions, to thereby establish the rest or non-print positions for those elements.
The construction shown by such references demonstrates the desirability in more recent print head designs of facilitating assembly of the print head while assuring continued alignment of the parts over extended periods of high speed operation.
However, further improvement remains desirable both in assembly of the print head and in assuring its precise and reliable operation over extended periods of time. In particular, because of the very rapid operation of the electromagnetic units and resulting movement of the armatures and print wires between their print and non-print positions, it has been found difficult to assure precise alignment of those components. Such alignment is of course essential in order to achieve uniform striking of the print wires as they approach their print positions. It has also been found difficult to limit undesirable generation of noise and heat during extended print head operation while at the same time assuring reliable operation.
Accordingly, there has been found to remain a need for further improvements in such wire matrix print heads which will further facilitate their assembly while assuring proper relative alignment and spacing between their operating components including print wires, armatures and electromagnetic units.